This invention relates generally to optical information processing, and in particular, to a parallel optical system for performing parallel optical arithmetic and logic operations.
There is a fundamental difference between optical circuits in which the information carriers are photons and electronic circuits wherein the information carriers are electrons. In the former case, the carriers do not interact with each other while in the latter case they do. This means that in optical devices there exist interconnection possibilities that do not exist with electronic hardware, in particular, interconnected parallel architectures. Furthermore, after inputs are switched on in an optical parallel architecture, a desired output appears in the time it takes a photon to traverse the optical device. No faster computation time is possible.
The evolution of optical computing in general has progressed from the analog Fourier transform type of optical computing to recent advances in numerical digital optical computing. Residue arithmetic numerical optical computing has also been investigated based on the compatibility between the parallel nature of arithmetic operations in residue arithmetic and the parallel processing capability of optics.
Briefly, the aforementioned capabilities lie in the fact that residue arithmetic does not have a "carry" operation, that is, each "bit" in the representation is independent of the other. Thus, for example, addition in residue arithmetic of corresponding "bits" in two numbers can effectively be carried out by a device that is not connected to other "bits", but parallel to other bits. In residue arithmetic, each "bit" in a representation of a number is the decimal value of the number modulo the prime number corresponding to that position, called the "radix". Addition is the sum of corresponding representations of the number modulo the radix. FIG. 1 gives examples of residue arithmetic in a "235" representation where 2 is the prime number associated with the leftmost radix, 3 with the central radix, and 5 with the rightmost radix.
Recently, a parallel optical adder for implementing residue arithmetic was proposed by Collins, et al. in "Optical Information Processing For Aerospace Applications II," NASA Conference Publication 2302, Aug. 30-31, 1983, incorporated herein by reference. Collins, et al. use controlled diffraction gratings produced by interfering light from two single mode optical fibers to implement residue addition. Referring to FIG. 2a, Collins et al, show a system, designated generally by reference numeral 10, that is provided with numerical information through two groups A & B of optical fibers 11 with the fiber ends acting as point sources in the input plane Q. The central two fibers 11a and 11b represent zero in each group A & B, respectively. Numerical values for input A increase in one direction from the central axis 20 (positive y direction) while numerical values for input B increase in the opposite direction. Light from two illuminated fibers, one in group A and another in group B, passes through a collimating lens 13 to produce interfering plane waves at the input side of a liquid crystal light valve 15.
The light valve may, for example, be the Hughes liquid crystal light valve more particularly described in W.P. Bleha et al "Application of the Liquid Crystal Light Valve to Real-Time Optical Data Processing," Opt. Eng., 17, p. 371.
The light valve 15 and its operation will be further described with reference to FIG. 2b. The control area 70 serves as an active area. An incident write beam 72 strikes a photosensitive CdS film (not shown) on the input side of the light valve 15. The CdS film allows the intensity of the write beam 72 to control the field produced across the liquid crystal layer. This in turn effects a change in the orientation of the liquid crystal molecules and hence provides control over their birefringence. This control is available at each of 600.times.600 image elements of the device simultaneously and independently. A polarized read beam 74 incident on the output side of light valve 15 is reflected by a dielectric mirror behind the liquid crystal layer. The intensity of the reflected beam 76 is thus controlled by the intensity of write beam 72 at any particular image element.
Thus, the interference pattern fringe spacing is a function of the distance between the illuminated fibers which, in terms of quantized units of fiber separation, is proportional to the sum of the two numbers represented by the corresponding fibers. For example, the distance between a fiber 11c representing the number 4 in Group A (four units from the 0 in Group A) and a fiber 11d representing the number 3 in Group B (three units from the 0 in Group B) is 8 units, seven units from the sum of 4 and 3, plus one unit representing the separation between the two 0's at 11a and 11b. On the output side of the light valve 15, light incident from a source 19 is directed toward the light valve via lens 25 to form the read beam 74 of FIG. 2b. This read beam is subsequently reflected by the dielectric mirror in light valve 15 and diffracted by the gratings resulting from the incident interference pattern on the light valve input side. The reflected and diffracted light is then focused through Fourier transform lens 25 onto Plane P into various diffracted orders of which only the +1 order is retained. The deflected distance from the z axis (i.e., along the y axis) of the diffracted spot 23 is proportional to the separation of the fibers 11 in the input plane. For example, the addition of two 0 inputs gives an output deflection of one unit. The deflected distance past this unit offset is equal to the sum of the two numbers represented by the fibers producing the initial diffraction grating, thereby yielding the result of residue operation.